In petroleum oil, in water and in gas pipelines there are from time to time leaks of the transported fluid through punctures or cracks in the pipe wall. Such leaks can be of economic and environmental importance and need to be located as quickly as possible. The more accurately they can be located the less disruption there is to the environment and to above ground infrastructure such as roads. There are means available to detect leaks of this nature but none of them is entirely successful in pipelines exceeding 300 mm in diameter or in plastic pipes exceeding 400 m in length between access points.
A sudden rupture can generate pressure transients that can be detected at each end of the pipe. The timing of these transients can be interpreted to give the location of the leak. However such transient detection means can be thwarted by attenuation in the pressure pulse over distances exceeding 400 m. Also smearing of the original signal by dispersion in pipes greater than 300 mm in diameter can invalidate interpretation. Dispersion is a physical phenomenon whereby the speed of sound is frequency dependent. As a result the characteristic shape of time varying signals are distorted with increasing distance. Dispersion can be a significant problem in some pipelines over distances over distances exceeding 400 m and would deny the benefit of the interpretation of pressure transients. Pressure transients can also arise from other sources than leaks and it is difficult to avoid false alarms. For these reasons, pressure transient detection is only used in limited application such as process plant.
A leak of fluid from a pipe that is under pressure generates broad-band noise that travels both upstream and downstream along the pipe and the contained fluid. If the noise is detected both upstream and downstream by means of a vibration transducer attached to the pipe then cross-correlation of these signals can often give an indication of the location of the leak. This means is a first stage in detection indicating there is a leak and giving some indication of the area in which it might be found. However this means is thwarted by attenuation particularly in plastic pipes exceeding typically 400 m in length and by dispersion in pipes typically 300 mm in diameter. A number of systems utilising vibration transducers in such a method are available from Palmer Environmental Ltd, Gutermann and Primayer Ltd for example.
The acoustic noise from a fluid leak is often transmitted through the surrounding material to the ground surface. The surface noise level is extremely low but there are means of detecting that noise and locating the leak. However the use of surface noise to give a final accurate location of a leak is a second stage location method. In the first stage one needs to know there is a leak to look for and to know roughly the area in which to look, otherwise a search can take a long time. Second stage devices that detect at a single point on the ground surface are known as ground microphones and are available from the companies mentioned previously. A second stage device that detects along a line rather than at one point is the Magic Carpet manufactured by Stest Ltd.
A first and second stage device for water pipes is available from P11 Waterline Ltd sold under the Registered Trade Mark of the ‘Sahara Leak Detection System’. This device consists of a hydrophone tethered to an umbilical cable that connects the hydrophone through the pipe back to the surface through a gland at the insertion point. The device is inserted into a pipeline and drawn along by the water. It drags the umbilical along behind it and listens for acoustic noise from leaks. The noise data is relayed back to the surface via the umbilical cable. The umbilical limits the distance over which this device can be deployed, typically up to half a kilometer. The use of an umbilical may disturb deposits on the pipe wall and so discolour the water-something that Water Companies prefer to avoid.
When distances are greater than a few hundred meters and the pipe diameter is greater than of the order of 300 mm it is more effective to detect leaks from the inside of the pipe. Such systems are first and second stage detection devices, determining that there is a leak and locating that leak. The Sahara System is limited in range of operation by the use of an umbilical.